Affiliate disclosure: Private Well Guide (privatewellguide.com) earns a commission from some links on this page, at no extra cost to you. We don’t test water or products ourselves. This article summarizes EPA and state agency guidance.
Health note: For radon testing and treatment decisions, contact your state radon office. This is not medical advice.
Why radon in water matters
Radon is a radioactive gas that forms as uranium in rock and soil decays (Wisconsin DNR). Groundwater can carry dissolved radon into your home. According to EPA’s Consumer’s Guide to Radon Reduction, radon in water poses an inhalation risk and a small ingestion risk. Most of the risk comes from radon released into the air during showering and other household water use.
EPA also notes that radon entering through the soil is usually a much bigger risk than radon from water. That’s why private well owners are advised to test both the air and the water.
Is there a legal limit?
There’s no federal drinking water standard for radon. Radon isn’t on EPA’s list of regulated drinking water contaminants. In 1999, EPA proposed a standard for community water systems, with options of 300 or 4,000 pCi/L depending on whether the state ran an indoor-air radon program. EPA’s own materials still describe it as a proposal, and it was never meant to cover private wells; EPA doesn’t regulate them (EPA archive).
States set their own guidance. Wisconsin DNR, for example:
- Advises reducing indoor air radon to 4 pCi/L or less
- Suggests considering water treatment if well water radon is over 4,000 pCi/L
- Cites an EPA estimate that a lifetime of household use of water with 20,000 pCi/L carries a 1-in-100 cancer risk, the same as breathing air at 4 pCi/L
Ask your state radon office what level it uses.
Should you test?
Test if:
– Your home’s indoor air has elevated radon, or your region is known for radon (EPA)
– Your well draws from granite or similar formations. Wisconsin DNR says the highest radon concentrations there are in groundwater flowing through granite or granitic sand and gravel.
– You’ve never tested. Wisconsin DNR notes you can’t know your well’s radon level without testing.
How: Testing water for radon uses different devices and procedures than testing air (EPA). Get a water-specific kit and instructions from your state radon office or a certified lab (EPA lab directory). Wisconsin, for example, points residents to its radon information centers and state lab of hygiene. Check that the kit is specifically for radon in water, not air.
Treatment: aeration vs GAC
For radon, EPA says to treat all the water entering the house (point-of-entry). Point-of-use devices at a single tap don’t reduce the risk from radon released by water used throughout the home (EPA).
| Aeration | Granular activated carbon (GAC) | |
|---|---|---|
| Typical radon reduction (EPA) | 95–99% | 85–95% |
| Best for | Higher levels | Moderate levels, around 5,000 pCi/L or less |
| Downsides (EPA) | Needs annual cleaning; must vent radon outdoors | Radioactive by-products build up on the carbon; may need shielding around the tank and careful disposal |
| Cost | Usually more than GAC | Usually less than aeration |
EPA recommends discussing the trade-offs with your state radon office or a water treatment professional. Many states keep lists of certified radon mitigation contractors (EPA state radon contacts).
Treatment overview: Well Water Treatment Options: UV vs RO vs Softener vs Whole-House Filter
Fix the air first?
Because soil-gas radon is usually the larger source, EPA suggests looking at both air and water results together to choose the right mitigation. Your mitigation contractor can help sequence the work.
While you’re testing
Radon testing is separate from your annual baseline (coliform, nitrate, pH, TDS). If you’re due for both, call your county lab or use a mail-in well panel for the baseline Tap Score: Essential well water test →. That panel doesn’t cover radon. How Often Should You Test Well Water? Annual, After Floods, Before Buying
Sources
- EPA, A Consumer’s Guide to Radon Reduction (2016): https://www.epa.gov/sites/default/files/2016-12/documents/2016_consumers_guide_to_radon_reduction.pdf
- EPA, National Primary Drinking Water Regulations: https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
- EPA archive, Radon in Drinking Water Q&A: https://archive.epa.gov/water/archive/web/html/qa1.html
- EPA, Protect Your Home’s Water: https://www.epa.gov/privatewells/protect-your-homes-water
- EPA, State radon contacts: https://www.epa.gov/radon/epa-map-radon-zones-and-supplemental-information
- Wisconsin DNR, Radon in private well water: https://dnr.wisconsin.gov/topic/Wells/radon.html
Private Well Water Testing: The Complete Guide for Rural Homeowners (2026) · How to Read Your Well Water Test Results (MCLs, ppm and ppb Explained)